Hypoplastic anemias and primary polycythemias often develop because some multipotential hematopoietic system cells inappropriately develop into mature red blood cells. To determine the reason(s) for this failuue, it is necessary to identify and characterize undifferentiated stem cells committed to later formation of erythrocytes. The main obstacle to this task has been the inability to identify these progenitor cells in vitro or in vivo. To approach this problem we propose: 1. To obtain highly purified and biologically active erythropoietin, the hormone that regulates the production of red blood cells. 2. To radioiodinate the purified hormone without altering its biological activity. 3. To employ the labeled hormone in experiments designed to identify and characterize the "erythropoietin-responsive cell(s) (ERC)". 4. To test the hypothesis that erythropoietin affects maturing cells of the erythroid series as well as the progenitor cell.